300SD running out of power- 72 mph top end?! - PeachParts Mercedes-Benz Forum

RobTheMod · #1 09-11-2006, 10:11 PM

My old SD has always proceeded around in a dignified manner... I know that she is not a rocket ship. Oil consumption is hanging around 1 qt per 1000 miles, with a few noticeable leaks. Oh well, 246,000 miles of which I've only done 24k. But on a highway trip I took last night...

What's up with this? I habitually cruise at 68-70 mph in this car, it just feels settled and relaxed. Still does, but on level ground, a foot to the floor got me 72 mph, and that, as they say, was that.
Hmmmm?

I know the blow-by is getting noticeable, but she did a quart per thousand back last year when she also did a verified 103 mph!

Compression test, possibly followed by tears, tomorrow. Is there anything else to look at? I noticed no visible smoke when flat out, that seems to me that I was completely "fueled out" and had a clean burn. Can filters cause this top-end "rev limiter"? I've seen the car get much slower in town when the filters got clogged, but never this...

#2 09-11-2006, 10:17 PM

Fuel filters ... loss of top end power is a classic symptom of clogged filters, that's when the engine needs the most fuel. I've had it happen a couple of times.

t walgamuth · #3 09-11-2006, 10:19 PM

not to be mr obvious but have you checked your filters?

also could be slipped fuel timing or bad fuel.

i doubt it is lack of compression. a quart every thousand does not correspond with that type of performance. i am presuming it used to go faster.

tom w

pawoSD · #4 09-11-2006, 10:30 PM

I agree, replace fuel filters and check the fuel tank strainer if there is still a problem with new fuel filters. MB technically says to replace them every 15-20k, so it may be due....all depends on how dirty the diesel is in your area etc....I usually get about 18-20k before mine get dirty and icky.

RobTheMod · #5 09-11-2006, 11:32 PM

Thanks for the encouragement, I was thinking about filters already, but I almost couldn't believe the way it was running... new filters going in tomorrow, then maybe a compression test. She's always been leaky and liked to swig some oil, but strafed Chiriaco Summit on I-10 in CA last year at 65-70, handily passing every large vehicle and some little pipsqueaks on the hill. She could get up to just over 100 (hard to tell with the 1981 speedo) and buried the needle three weeks ago.

pawoSD · #6 09-11-2006, 11:40 PM

Quote:

Originally Posted by RobTheMod

Thanks for the encouragement, I was thinking about filters already, but I almost couldn't believe the way it was running... new filters going in tomorrow, then maybe a compression test. She's always been leaky and liked to swig some oil, but strafed Chiriaco Summit on I-10 in CA last year at 65-70, handily passing every large vehicle and some little pipsqueaks on the hill. She could get up to just over 100 (hard to tell with the 1981 speedo) and buried the needle three weeks ago.

Yep, definatley a fuel issue from the sound of that, the loss of power from a compression problem is extremely gradual, and would never have occured in a matter of weeks. Plus, and engine with a bad compression problem that would affect performance would also be quite difficult (or impossible) to start up when cold. I'd make sure all the fuel filters/fuel system are top notch first, then maybe check the ALDA boost line from the manifold to make sure its not clogged with anything, which would prevent upper end boost/power due to lack of fuel. Its 99.9% likely that one of those things is the issue. Compression check is a very last resort.

RobTheMod · #7 09-11-2006, 11:51 PM

I'll check the ALDA line too, I do run a Dawes boost controller and she still pulls about 11 psi all out so I guess I wasn't thinking of that. As for cold starts there's no such thing in AZ in September, but she doesn't even need a glow, fires off with a turn of the key first thing in the AM... so I guess compression would be silly to worry about then, but I was panicking as I roared along flat-out while semis passed me- thought I had a dead cylinder with 4 good ones, or something, but that isn't how it sounds- (I've heard a 617 with a dead miss, that's not like mine)

vstech · #8 09-12-2006, 08:31 AM

have you recently driven through some really dusty conditions?
a badly plugged air filter can cause similar reduction in power.
John

Lostyankee · #9 09-12-2006, 12:10 PM

In one of my mismodifications, I added some lenght to the pre-filter line (the old one was kinked) & at certin conditions this new line would 'flop' up and lodge against the steel fuel line. As the pump operated and my foot did the up & down in traffic, the line kinked and I lost RPMs. After fixing this, I blew out the flue line and found some crud (another gift from the PO). I do do WVO & now blow out my lines once a year. Before the final fix, car would roar to 80 MPH and after a few minutes, top end was 45 MPH!
ALDA truned too far in will cut back fuel.
With your oil consumption: have you concidered a runaway diesel? CO2 fier ext. is a good investment.
Have you done a purge?

rocketman93116 · #10 09-12-2006, 12:15 PM

check the banjo bolt orifices on back of intake manifold, could be plugged, clean it out however it looks, use a wire paper clip and brake clean.

Hatterasguy · #11 09-12-2006, 12:48 PM

You are asking for full fuel flow at those speeds, marginal fuel filters show themselves at top end first. I'd change them first, than move on to the other more expensive stuff if that doesn't do it.

Brian Carlton · #12 09-12-2006, 06:22 PM

I had an interesting experience with the SD today.

Was fairly low on fuel..........between the 1/4 mark and the R.............and it developed a lumpy idle........like one cylinder had insufficient fuel. Hmmmm..........

Took it out on the highway and upon acceleration up the ramp, I knew something was wrong..........definitely missing the power. A few more miles and it wouldn't hold 60 mph on grade.

After about 10 miles, I took it for fuel...........still with a horrible idle..........put exactly 17 gallons to the top of the filler pipe.............went exactly 500 miles.

Idle returns to normal and power returns to normal.

Now, I'm fairly convinced that this vehicle has some bugs in the tank due to sitting for the greater part of one year during most of 2005. It also shows some fungus in the prefilter occassionally. But, why the fuel has flow problems when the tank is low is definitely a quandry.

In theory, there was an additional 3 gallons in the tank.

Hatterasguy · #13 09-12-2006, 07:23 PM

I ran my SD out of fuel and it took just shy of 18 gallons to fill it back up.

RobTheMod · #14 09-12-2006, 10:22 PM

But, I don't think it burns all the oil it loses. This one *really* likes to leak, from the VC gasket, seeps at the oil pan gasket, also at the turbo and oil separator drains @ the pan. Just haven't gotten to it yet, I know that I can keep it full of oil and I check it at each fill up. If it was drinking enough oil to runaway, I don't think I'd have enough compression to start without glowing it, and I still do...

t walgamuth · #15 09-12-2006, 11:43 PM

brian, as a person who has coasted into gas stations on fumes more than once, missing a bit is normal when running out of fuel.

but maybe in your case the dirty fuel thing is more appropriate diagnosis. if you were running out you prob couldnt have gotten far enough to do tests.

time for fungicide i suppose.

tom w